Japan on Thursday announced its first ban on rice produced near the crippled Fukushima nuclear power plant after samples showed radioactive contamination well above legal limits.

The findings will further worry nervous consumers, already fretting over the safety of domestic produce, despite its previous solid safety reputation.

Authorities in Fukushima prefecture say rice produced near the stricken atomic power plant contained caesium they measured at 630 becquerels per kilogram (2.2 pounds). The government safety limit is 500 becquerels.

Chief Cabinet Secretary Osamu Fujimura ordered Fukushima Governor Yuhei Sato to restrict shipments of rice from Onami - from where the samples were sourced - according to an agricultural ministry official.

"This restriction won't be lifted until safety of the rice produced in the area can be confirmed," the official said, adding that the ban will affect 154 farms that produced 192 tonnes of rice this year.

It is the first ban on rice shipments since the Fukushima Daiichi nuclear power plant was damaged by a massive quake and tsunami on March 11, when cooling systems failed and radiation was spewed into the air, oceans and food chain.

While the natural disaster claimed 20,000 lives, the nuclear emergency has recorded no direct casualties, but it has badly dented the reputation of a technology on which Japan previously depended for a third of its electricity.

Bans on food following the crisis are nothing new but rice - eaten three times a day in many homes - holds a special place in the Japanese heart.

Japanese-grown grain is widely held to be superior to imports, and is heavily protected by massive tariffs aimed partially at propping up the nation's ageing farmers.

The polluted samples were taken at a farm in Onami, 57 kilometres (35 miles) northwest of the troubled plant.

None of the 840 kilogrammes of rice produced at the farm this year has been shipped to markets, local officials said.

The price of rice produced before the disaster temporarily shot up in the summer as people rushed to stock up over fears this year's harvest would be contaminated. Prices have since stabilised.

A team of international researchers this week said elevated levels of caesium in soil in the region would 'severely impair' food production in eastern Fukushima.

The study, published in the US-based Proceedings of the National Academy of Sciences journal, suggested farming in neighbouring areas could also suffer because of radiation.

Shipments of a number of farm products from affected regions were halted as the crisis unfolded and even those that were not subject to official controls have found little favour with Japanese consumers wary of the potential health effects.

Environmental campaign group Greenpeace said Thursday it had detected radiation in fish sold at Japanese supermarkets, although radiation levels were still well below the government safety limit of 500 becquerels.

According to its own research carried out between October 12 and November 8 in eastern Japan, 47.3 becquerels of cesium per kilogramme were discovered in cod while traces of radiation were also found in other fish, including tuna.